An ode to the many evolved virtues of human semen

Dear readers: I have come upon a secret treasure, a heretofore-unknown bounty of facts only recently unearthed by a team of evolutionary psychologists. A vital forewarning, though: although the data and information I am about to share oozes with the promise of dramatically improving virtually every aspect of your wellbeing, it can also be abused with tragic—even fatal—consequences. This is so much the case, in fact, that I debated the merits of popularizing this material and do so here only with great circumspection and caution. So please be wise in digesting this semen-related knowledge, and be wiser still in applying it to your own sex lives.

As with the origins of so many great scientific discoveries, this story begins with a serendipitous chain of events. “Our interest in the psychological properties of semen arose as a by-product of an initial interest in menstrual synchrony,” explain co-discoverers Gordon Gallup and Rebecca Burch, evolutionary psychologists from the State University of New York system, in a 2006 chapter about human semen. In particular, Gallup and Burch had stumbled onto a set of intriguing data from the mid-1990s showing that, unlike heterosexually active women residing together, sexually involved lesbians failed to exhibit the well-known “McClintock effect,” in which menstrual cycles in cohabitating women (as well those of females from many other species) are synchronized. Since subtle olfactory cues (called pheromones) are known to mediate menstrual synchrony, write the authors, “This struck us as peculiar...”

… because lesbians would be expected to be in closer, more intimate contact with one another on a daily basis than other females who live together. What is it about heterosexual females that promotes menstrual synchrony, or conversely what is it about lesbians that prevents menstrual synchrony? It occurred to us that one feature that distinguishes heterosexual women from lesbians is the presence or absence of semen in the female reproductive tract. Lesbians have semen-free sex.

Perhaps you already see where this is leading. Gallup and Burch reasoned that certain chemicals in human semen, through vaginal absorption, affect female biology in such a way that women who have condomless sex literally start to smell different from those women who do not—or at least, their bodies emit the pheromones that “entrain” menstrual cycles among cohabitating women. (Their hunch was indeed borne out by reviewing the existing literature on menstrual synchrony.) But this happenstance discovery of asynchronous lesbians was just the tip of the semen iceberg for Gallup and Burch, who quickly discovered that, although much was known among biologists about basic semen chemistry, virtually nothing was known about precisely how these chemicals might influence female biology, behavior and psychology.

And that is a rather odd omission in the biological literature indeed, since there could hardly be anything more obvious in Darwinian terms than the fact that semen is, almost by definition, naturally designed to get into the chemically-absorptive vagina. Bear in mind that although they are often conflated in everyday parlance, along with many other less scientific terms, “semen” is not the same thing as “sperm.” In fact, you may be surprised to learn that only about 1 to 5 percent of the average human ejaculate consists of sperm cells. The rest of the ejaculate, once drained of these famously flagellating gametes, is referred to as “seminal plasma.” So in discussing the chemical composition of semen, it is the plasma itself, not the spermatozoa, that is at issue.

Now, medical professionals have known for a very long time that the vagina is an ideal route for drug delivery. The reason for this is that the vagina is surrounded by an impressive vascular network. Arteries, blood vessels, and lymphatic vessels abound, and—unlike some other routes of drug administration—chemicals that are absorbed through the vaginal walls have an almost direct line to the body’s peripheral circulation system. So it makes infinite sense, argue Gallup and Burch, that like any artificially-derived chemical substance inserted into the vagina via medical pessary, semen might also have certain chemical properties that tweak female biology.

It turns out that this insight, so obvious as to be all but invisible, has been a theoretical gold mine for this hawkeyed pair of adaptation-minded thinkers. But before we jump into their rich vat of seminal theory, let’s have a quick look at some of the key ingredients of human semen. In fact, semen has a very complicated chemical profile, containing over 50 different compounds (including hormones, neurotransmitters, endorphins and immunosupressants) each with a special function and occurring in different concentrations within the seminal plasma. Perhaps the most striking of these compounds is the bundle of mood-enhancing chemicals in semen. There is good in this goo. Such anxiolytic chemicals include, but are by no means limited to, cortisol (known to increase affection), estrone (which elevates mood), prolactin (a natural antidepressant), oxytocin (also elevates mood), thyrotropin-releasing hormone (another antidepressant), melatonin (a sleep-inducing agent) and even serotonin (perhaps the most well-known antidepressant neurotransmitter).

Given these ingredients—and this is just a small sample of the mind-altering “drugs” found in human semen—Gallup and Burch, along with psychologist Steven Platek, now at the University of Liverpool, hypothesized that women having unprotected sex should be less depressed than suitable control participants. To investigate whether semen has antidepressant effects, the authors rounded up 293 college females from the SUNY-Albany campus, who agreed to fill out an anonymous, written questionnaire about various aspects of their sexual behavior. Recent sexual activity without condoms was used as an indirect measure of seminal plasma circulating in the woman’s body. Each participant also completed the Beck Depression Inventory, a commonly used clinical measure of depressive symptoms.

The most significant findings from this 2002 study, published with criminally modest fanfare in the Archives of Sexual Behavior , were these: even after adjusting for frequency of sexual intercourse, women who engaged in sex and “never” used condoms showed significantly fewer depressive symptoms than did those who “usually” or “always” used condoms. Importantly, these chronically condomless, sexually active women also evidenced fewer depressive symptoms than did those who abstained from sex altogether. By contrast, sexually active women, even really promiscuous ones, who used condoms were just as depressed as those practicing total abstinence. In other words, it’s not just that women who are having sex are simply happier, but instead happiness appears to be a function of the ambient seminal fluid pulsing through one’s veins.

And it gets better. A smaller percentage (4.5 percent) of the sexually active women who “never” used condoms were less likely to have attempted suicide than were those who “sometimes” (7.4 percent) and “usually” (28.9 percent) and “always” (13.2 percent) used condoms.

Relax, settle down, take a deep breath—I know what you’re thinking. This is a correlational study and there are scores of possible confounds, both those that the authors anticipated and controlled for in this study design (by all means read the actual article for more details—but please do note that these between-group differences in depression panned out even after controlling for the use of oral contraceptives, days since last sex, frequency of sex and duration of the relationship with the male partner) and probably some that you can come up with on your own. Even the authors urge some degree of scepticism:

It is important to acknowledge that these data are preliminary and correlational in nature, and as such are only suggestive. More definitive evidence for antidepressant effects of semen would require more direct manipulation of the presence of semen in the reproductive tract and, ideally, the measurement of seminal components in the recipient’s blood.

Now I’m hedging here, because there are all sorts of equally fascinating directions I can go from this point. But one thing I do want to mention, with a helpful nod from the authors of this study, is that the antidepressant effects of seminal plasma may not be limited to vaginal absorption of its mood-brightening chemical properties. “It would be interesting to investigate,” write Gallup and his coauthors, “the possible antidepressant effects of oral ingestion of semen, or semen applied through anal intercourse (or both) among both heterosexual couples as well as homosexual males.”

So in my plumbing of the empirical literature for studies on unprotected anal sex among gay males, otherwise known as “barebacking,” I came across a load of research on this very topic. Most of this work is couched, understandably so, in the HIV-prevention literature. One particularly telling study, though, comes from a 2005 report from the journal Nursing Inquiry , in which Canadian investigators Dave Holmes and Dan Warner interviewed barebacking gay males—not while they were engaged in the act, but through later introspection—about their motivations for preferring unprotected anal sex over condoms in light of the obvious dangers of infection. The most intriguing result to emerge from this study, in the context of Gallup and Burch’s overall theoretical perspective regarding the psychobiology of semen, was that so many of the barebacking interview subjects viewed the exchange of semen through unprotected anal sex as providing them with a palpable sense of “connectedness” with their same-sex partners, one that happened only with the internally, unimpeded ejaculation.

Unfortunately, rather than investigate the possible psychobiological effects of semen exchange in this dynamic, Holmes and Warner leer through a fairly typical postmodernist lens to explore the symbolic nature of semen exchange in barebackers. Now, I ask you, which is the more informative paradigm for understanding why gay men would practice unsafe sex through unprotected anal intercourse: an evolutionary biological account taking into consideration the chemical composition of seminal plasma and its possible affects on attachment among gay men, or a symbolic, postmodernist perspective like the following one advanced by Holmes and Warner (in all fairness, this is just a snippet, but a good taste of their approach):

The body becomes the locus of never-ending fights, a carnal battlefield. The escape route (lines of flight) is intrinsic to the deterritorialization of the Body-without-Organs through which one becomes someone else. However, the lines of flight could have paradoxical effects. Indeed, they can be avenues of creative potential or, conversely, paths of great danger. Yet, it is ‘always in a line of flight that we create’ … ‘that we must continue to experiment with such lines.’ Lines of flight (nuclei of resistance of resingularization and heterogenesis) permit freedom to surge through a process of creative transformation and metamorphosis.

Trust me, even in context that paragraph reads like the authors were cobbling together a braille sentence using the random distribution of acne on someone’s back. Sorry to sound a bit testy, but while such soupy postmodernist rhetoric may still have its place in certain scholarly circles, in dealing with something as clinically important as unprotected sex among vulnerable populations, a scientific understanding of these people’s motivations is essential before any intervention of their high-risk behaviors can even begin to occur.

You may also be beginning to realize the dangers that I alluded to at the start of this essay. For both men and women, heterosexual and homosexual, knowing that the penis is capable of dispensing a sort of natural Prozac—whether obtained vaginally, anally or orally—without also considering the viral arms race involving sexually transmitted infections, can lead to very tragic decisions indeed and many undocumented high-risk private bedroom “experiments.” But here’s just one reason to put the brakes on such plans: The HIV-virus, which evolved long after these adaptive antidepressant factors, has apparently come to pirate human semen, such that certain protein factors in seminal plasma, particularly a protein called prostatic acid phosphatase , make HIV up to 100,000 folds more potent than it is outside of the plasma.*

In any event, Gallup and Burch’s model reminded me also of those oft-cited Papau New Guinea tribes, such as the “Sambia,” and their semen-ingestion rituals involving young boys. On the surface, there’s a puzzling scenario here: such cultures have long histories of being embroiled in violent warfare and thus they tend to place extraordinarily high value on expressed masculinity. Yet ritualized homoerotic practices involving young boys fellating older males in order to ingest their semen are common. In a 2000 issue of the Archives of Sexual Behavior, Gilbert Herdt, a cultural anthropologist who studied the Sambia, along with his colleague Martha McClintock (the same McClintock named for the menstrual cycle synchrony effect discussed earlier), describe how “… by the age of 11–12 (Sambia) boys have become aggressive fellators who actively pursue semen to masculinize their bodies.”

In the past, this semen ritual has been conceptualized by cultural anthropologists such as Herdt mostly in symbolic terms. Yet, just a hunch, but since testosterone from the seminal plasma could penetrate the oral mucosa, along with a surfeit of other hormones and chemicals having possible spin-off effects on male behavior, it is not inconceivable to me that there may be genuine psychobiological consequences of semen intake occurring in these young swallowing males that are not wholly out of line with the Sambia’s own folk beliefs. It might not be a theory you want to run by your local pastor or should bring up at your next PTA meeting, but you get the idea.

But let’s get back to non-hebephilic semen ingestion. In addition to their semen-as-antidepressant model, Gallup and Burch have worked out many other intricate, persuasive arguments about how the various chemicals in human semen served—and continue to serve—biologically adaptive functions for both sexes. For example, among the more curious ingredients in human semen are follicle-stimulating-hormone (FSH) and luteinizing hormone (LH). The reason this is curious, point out Gallup and Burch, is because these are distinctively female hormones. “What are female hormones doing in human semen?” The authors speculate, and convincingly so, that the presence of FSH and LH in human semen is related to concealed ovulation in human females.

Unlike females of other primate species, women do not have breeding patterns governed by season or standardized cycles, and there are no obvious signals—such as a fire-engine red, swollen rear end—giving away their time of the month. So for a naïve human male, impregnating a woman as a consequence of sexual intercourse is much more a roll of the dice than it is for males of other species in their mating behaviors. Just as with any other species, though, getting the timing right so that release of semen coincides with the release of eggs is key. As a counterdefense against women’s concealed ovulation, male evolution had a trick up its sleeve, which was the ability to manipulate the timing of a woman’s ovulation to suit his own insemination schedule—that is to say, semen chemistry seems to give premature eggs a nice little nudging. Hence the conspicuous presence of FSH (which causes an egg in the ovary to ripen and mature) and LH (which triggers ovulation and release of that egg).

In support of this theoretical claim about semen chemistry and concealed ovulation in human females, consider that chimpanzee semen lacks the FSH hormone altogether and the presence of LH is rather negligible, which makes sense, of course, since chimpanzees are cyclical breeders and ovulating females display blinking Vegas-style marquees by way of swollen, multicolored anogenital regions. “Thus it would appear,” reason Gallup and Burch:

…that the chemistry of human semen has been selected to mimic the hormonal conditions that control ovulation, and as such may account for instances of induced ovulation (ovulation triggered by copulation at points in the menstrual cycle when ovulation would otherwise be unlikely).

I’ve barely scratched the surface of the evolved semen literature. Here’s a snapshot of other recent findings from Gallup’s lab: semen-exposed women perform better on concentration and cognitive tasks; women’s bodies can detect “foreign” semen that differs from their recurrent sexual partner’s signature semen, an evolved system that, Gallup believes, often leads to unsuccessful pregnancies because it signals a disinvested male partner who is not as likely to provide for the offspring; women who had unprotected sex with their ex-partners—and therefore were getting regularly inseminated—experience more significant depression on breaking up than those who were not as regularly exposed to their ex’s semen (and they also go on the “rebound” faster in seeking new sexual partners, which presumably would help fix their semen-deprived depression). And the list goes on.

Before I bid adieu, please accept, in all sincerity, my humblest apologies for what is likely to be a flood of bad, off-color jokes—men saying, “I’m not a medical doctor, but my testicles are licensed pharmaceutical suppliers” and so on—tracing its origins back to this innocent little article. Ladies, forgive me for what I have done.

*Editors' Note, 9/28/10: The word "brakes" in this paragraph was originally mistyped as "breaks." Thanks to the readers who pointed out the error.

The views expressed are those of the author(s) and are not necessarily those of Scientific American.

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ABOUT THE AUTHOR(S)

Jesse Bering

Jesse Bering is Associate Professor of Science Communication at the University of Otago in New Zealand. He is the author of The Belief Instinct (2011), Why Is the Penis Shaped Like That? (2012) and Perv (2013). To learn more about Jesse's work, visit www.jessebering.com or add him on Facebook (https://www.facebook.com/jesse.bering).

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